Optical circulator devices are well known in the art and normally comprise a series of bi-directional ports and a "non-reciprocal" mapping between ports. For example, in a three-port optical circulator device, the ports may be designated A, B and C and the non-reciprocal nature of the device is such that an input signal at Port A will be output at Port B, an input signal at Port B will be output at Port C and an input signal at Port C will be output at Port A.
It is desirable with any circulator type device to manufacture as compact and inexpensive a device as possible.